Monolithic integration of optical components on a III-V semiconductor is emerging as an important field known as photonic integrated circuits (PICs). In particular, photonic integrated circuits consist of active and passive optical components fabricated on a single III-V semiconductor substrate. In addition to the realization of new functional devices, such as modulators, switches, splitters, lasers, and detectors, photonic integrated circuits simplify device packaging and testing.
Recently, the inventors have demonstrated the material compatibility of the fundamental building blocks for PICs by monolithically integrating on a single chip a distributed feedback (DFB) laser, passive Y-junction waveguide, and p-i-n photodiode. See K. Y. Liou et al., Appl. Phys. Lett., Vol. 54, No. 2 pp. 114-6 (1989). In the above device by Liou et al., as well as in most PICs, the Y-junction waveguide, which interconnects active devices by dividing an incident optical signal into two output branches, is an indispensable waveguide component. As such, it is not surprising that various Y-junction designs have been fabricated on various substrates, such as glass, lithium niobate and gallium arsenide (GaAs). See, for example, U.S. Pat. Nos. 4,674,827, 4,846,540, and 4,850,666.
Although prior art Y-junction waveguides perform acceptably, due to limitations of fabrication techniques, practical Y-junction or branching waveguides deviate from their ideal designs, which in turn, has deleterious effects on optical devices connected thereto. For example, the wedge tip of a Y-junction waveguide typically becomes blunt, that is truncated, when processed by wet chemical etching techniques because of undercutting. Importantly, this truncation of the wedge tip at the Y-junction results in a substantial amount of optical back-reflection as well as radiative loss. See, for example, Sasaki et al., Electronics Letters, Vol. 17 No. 3 pp. 136-8(1989). Low back-reflection and low-loss characteristics of a Y-junction waveguide are particularly attractive for monolithically integrated active optical devices because their performance is highly dependent on the loss and reflectivity properties of the Y-junction. For example, distributed feedback (DFB) and distributed bragg reflector (DBR) lasers typically require optical isolation of better than 50 dB for stable single frequency oscillation. Further, for traveling-wave semiconductor laser amplifiers, optical isolation should be more than 40 dB in order to suppress ripples in the gain spectrum due to residual Fabry-Perot resonances. However, prior to the present invention, there have been no Y-junction or branching waveguides designed to minimize the effect of wedge tip truncation.